This invention concerns mechanisms for intermittently advancing a motion picture film along a film guide past the film aperture of a motion picture machine.
The invention relates primarily to intermittent movements for use in motion picture cameras, and will be so described for the sake of clarity. However, many aspects of the invention are useful in other types of motion picture machines, such as projectors, printers and the like.
The invention has to do more particularly with cam-driven intermittent movements in which a claw arm is driven in oscillation about a pivot axis and carries a claw structure in a definite closed path. That path includes a pull-down stroke generally parallel to the film guide with the claw engaging and driving the film, and a return stroke with the claw released from the film.
In some movements of that general type the meshing action by which the claw alternately engages and releases the film may be produced by ratchet action of the claw structure itself. However, it is generally preferred that the cam-driven movement of the claw arm includes path segments transverse of the film for positively inserting the claw in a film perforation prior to the pull-down stroke and for retracting the claw after completion of that stroke. Such meshing segments of the claw path are preferably clearly distinct from the film driving strokes in order to minimize sawing action of the claw on the perforation edge.
Such intermittent movements commonly use one or more self-conjugate cams, which have constant diameter and can thus work positively between pairs of parallel opposed cam followers mounted on the claw arm. Such cams have the great advantage that they can be designed to produce relatively rapid to and fro movements which are separated by dwell periods. For example, each stroke may occupy only 90.degree. of cam rotation relative to the cam followers. The drive strokes then alternate with 90.degree. dwell periods.
A single cam of that type with dwell periods of at least 90.degree. may cooperate with one pair of cam followers to drive the swinging movement of the claw arm about its pivot, producing the film driving claw strokes, and may also cooperate with a second pair of cam followers perpendicular to the first to drive the claw arm in longitudinal translational movement, producing the meshing action of the claw. Alternatively, two cams may be mounted on a common shaft, engaging respective pairs of suitably oriented cam followers. In either case, the cam rest periods permit the claw arm movements in one dimension to occur largely or wholly during the dwell periods of its movements in the other dimension. Hence the driving segments and the meshing segments of the claw movement are essentially distinct, producing typically a generally rectangular claw path.
With suitable cam design, the dwell periods can be extended beyond 90.degree., typically to as much as 120.degree., with corresponding shortening of the stroke periods and speeding up of the driven movements. That has the great advantage of reducing the fraction of each cycle that is occupied by the film movement, correspondingly extending the time available for exposing the film at the film aperture.
However, for a given cam throw, such extension of the dwell periods beyond 90.degree. is obtained at the cost of increased cam diameter. Extension of the film exposure time is therefore limited in practice by design problems of accommodating the larger cam size and by the tendency of the larger cam to produce more noise and vibration.
A primary object of the present invention is to reduce the inherent noise and vibration produced by cam-driven intermittent movements, especially those providing rapid pull-down.
The invention further permits the design of fully practical intermittent movements which retain the known advantages of self-conjugate cam drive and yet provide more rapid pull-down than has previously been feasible.
The invention further provides great flexibility of detailed design of cam-driven movements, especially with respect to the proportions and time relations of the claw path.
At the same time, the invention maintains the excellent reliability and virtually the full economy of construction for which conventional cam-driven movements are well known.